Photoconductive elements are composed of a conducting support having a photoconductive layer which is insulating in the dark but which becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in nonirradiated areas a charge pattern, referred to as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Numerous photoconductors have been described as being useful in electrophotography. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, azoles, carbazoles, pyrroles, phthalocyanines and the like.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (sometimes called multiactive elements) have at least one charge-generation layer and at least one charge-transport layer. Under actinic radiation, the charge-generation layer generates mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
The majority of known photoconductors are sensitive to ultraviolet and visible electromagnetic radiation. However, increasing use is being made of diode lasers which emit radiation principally in the near-infrared region of the electromagnetic spectrum, i.e., from 700 nm to about 900 nm. Known photoconductors either have little or no sensitivity to such radiation, or they have other disadvantages. For example, they may become increasingly conductive in the dark and lose their ability to hold an electrostatic charge (a process known as dark decay), or they may have poor quantum efficiency which results in low electrophotographic sensitivity, or they may require an extremely high electrostatic charge or other extreme conditions.
There is, therefore, a need for photoconductive elements sensitive to the near-infrared region of the electromagnetic spectrum and having low dark decay and high sensitivity.
Borsenberger et al in U.S. Pat. No. 4,471,039 have disclosed that when the .beta.-phase of an indium phthalocyanine pigment is used as the charge generation layer in a multiactive electrophotographic element, the element has high sensitivity in the near-infrared region.
Although the phthalocyanine pigments disclosed by Borsenberger et al have high infrared sensitivity and low dark decay, it has been preferred to purify them by a sublimation in order to obtain high speed and low dark decay, a purification step which can be costly.